Beginner: Faces unable to subdivide
The subdivide tool in Blender [W > Subdivide] mainly works on edges, but in a few special cases including the cases of triangles and quadrangles, it will also work on a face. However, the subdivide tool will not work on the class of faces called N-gons. To subdivide an N-gon either  manually divide the face into quadrangles and triangles, or enter edge select mode, and divide the edges around the face and subdivide the edges.